1. Field of the Invention
The present invention relates to a fixing carrier for fixing an activated sludge employed in a biochemical treatment of waste waters such as sewage or industrial waste waters containing organic matters.
2. Description of the Prior Art
Hitherto, in order to remove the organic matters from the waste water, a biochemical treatment of the waste water employing microorganisms has been employed. As this treatment, an activated-sludge treatment process is generally conducted: an activated sludge (an aggregation of microorganisms) is suspended and aerated in an aeration tank of a waste-water treatment apparatus so that pollutants contained in the waste water are decomposed and removed. However, such activated-sludge treatment process has problems that pollutant, loading per unit of aeration-tank volume is too small and that a large amount of excess sludge is produced.
These problems are resolved by a prior art described in Japanese Laid-Open Patent Publication No. 60-150893 relating to a fixing carrier for the activated sludge (microorganisms) employed in the biochemical treatment of waste water, which carrier is constructed of inorganic-foam pellets having a mean-particle diameter of nothing smaller than 2 mm and a specific gravity of up to 1.5. The pellets are prepared as follows: first, a foaming agent and a glass-forming agent are blended with expansible minerals, non-expansible minerals or refractories to prepare a mixture which is then powdered and shaped; and the pelleted mixture is baked into the above inorganic-foam pellets. In addition, in order to resolve the above problems, another proposal is made, in which crushed stones are employed as the fixing carrier for the activated sludge employed in the activated-sludge treatment process. Further another proposal is also made in a published document Yousui Haisui Handbook (2) "(Handbook (2) of Water and Waste Water)" published by Sangyo Yousui Chosa Kai "(A Board of Investigation of Industrial Waste Water)" on Nov. 30, 1974, pp. 823-824, FIGS. 3 and 4. This proposal relates to an activated-sludge treatment process in which a honeycomb-like fixed-bed type carrier made of plastics serves as a carrier for the activated sludge and is disposed in a biochemical-reaction tank of a waste-water treatment apparatus.
However, the above-mentioned prior art and proposals suffer from many problems.
For example, as for the inorganic-foam pellets disclosed in the Japanese Laid-Open Patent Publication No. 60-150893, a complex-production process thereof makes it difficult to provide such pellets at low cost. In addition, in the biochemical treatment of the waste water, foams or pores in the pellet are filled with water to increase apparent specific gravity of the pellet. This deteriorates flow properties of the pellet to cause the pellet so to be deposited in a dead space of the aeration tank or reactor. As a result, in such dead space, the activated sludge fixed to the pellets dies from lack of oxygen, accumulates to make the space anaerobic and rots to produce a harmful gases, for example such as hydrogen sulfide and the like. The hydrogen sulfide and like harmful gases deteriorate the functions of the activated sludge, so that a treatment performance of the aeration tank employing the above inorganic-foam pellets becomes remarkably poor. On the other hand, in case that the inorganic material is foamed to have an apparent specific gravity of up to 1.5, the pellet is greatly occupied by volume of the foams or pores while poor in its sturctural strength, which leads to breakdown of the pellet in its use for a long period of time.
In case that the crushed stone is employed as the fixing carrier for the activated sludge, the crushed stone is poor in both surface area to weight ratio and surface area to volume ratio. For example, as for a crushed stone having a diameter of 50 mm, its surface area to volume ratio is only 90 m.sup.2 /m.sup.3. In this case, due to a lack of an effective surface area for fixing the activated sludge (microorganisms), it is impossible to retain a sufficient amount of the activated sludge (microorganisms) in the biochemical-reaction tank so that a treatment efficiency of waste water by sue of such biochemical-reaction tank is remarkably low. Further, a porosity of the crushed stone is about 35%. Since this value is very small, voids or pores of the crushed stone are immediately clogged with the activated sludge having proliferated. In order to prevent the voids or pores of the crushed stone from being clogged with the proliferated activated sludge, it is necessary to frequently conduct a washing operation of the crushed stone serving as the fixing carrier for the activated sludge. Incidentally, the crushed stone has a defect that the porosity of the crushed stone decreases as the surface area of the same increases. In case that the honeycomb-like fixed-bed type carrier made of plastics and the like is employed as the fixing carrier for the activated sludge, the honeycomb construction of such carrier is readily clogged with the proliferated activated sludge. In addition, such honeycomb construction has a problem in that it is difficult to realize a steady activated sludge layer therein: the activated sludge once fixed o the honeycomb construction readily drops out of the construction to deteriorate a quality of a treated water.
As described above, since the conventional fixing carrier for the activated sludge employed in the sewage or waste-water treatment process has many problems, it is difficult to employ such conventional fixing carrier in a large-scale treatment of sewage and other waste water, for example such as: ammonia liquors discharged from coke ovens of ironworks; waste water discharged from coal-gasification or liquefaction plants; waste water produced in the petroleum-refining process; waste water discharged from food-processing plants; waste water discharged from fermentation plants of alcohol and the like; and like waste waters.